1. Field of the Invention
The present invention relates generally to fluid valve arrangements that permit flow under pump pressure and automatically close against flow when the pump is turned off. In one preferred aspect, the invention relates to mud saver valves of the type used in oil drilling operations. In other aspects, the invention relates to knockout caps useful for such mud saver valves.
2. Background of the Invention
It is standard practice in drilling operations to insert a mud saver valve between the kelly and the drill pipe in order to help prevent loss of drilling mud when the connection between the kelly and the drill pipe is broken. The recognized advantages of such valves include the saved cost of lost drilling mud, less pollution and greater safety for drilling rig personnel since less lost mud results in fewer slippery floors and surfaces in the rig.
Conventional mud saver valves incorporate a spring-biased check-valve or poppet-type valve that opens to permit mud flow downwardly into the drill pipe. When the mud flow is turned off, the spring biases the poppet valve closed so that mud cannot pass through the valve.
Unfortunately, conventional poppet-type mud saver valves usually need to be machined to close tolerances and may be susceptible to wear from the abrasive muds that are passed through them, particularly around the area of the valve seat. Over time, this wear can deteriorate the ability of the valve to seal. Also, if the seals of the poppet valve have a slight leak, the valve will likely not seal properly, and under pump pressure, the valve may begin throttling in an undesirable manner. The valve seat may also be vulnerable to impact damage.
In addition, under normal operating conditions when such a valve is open, turbulent flow develops through the valve body which leads to washing out or eroding of portions of the valve body. This turbulence results at least partially because fluid passing through these types of valves is directed radially outwardly through the space between the valve body and the valve seat, thus changing the direction of flow. Further, the flow is often directed toward and into the walls of the flowbore, creating further turbulence in the flow.
Vent caps are known for use in mud saver valves. These caps permit venting of excessive downhole pressure through the kelly valve. Some vent caps are designed to be broken away in the event that it is desired to pass tools downward through the mud saver valve. One such cap is disclosed in U.S. Pat. No. 3,965,980 issued to Williamson. In order to replace this type of cap, however, stop pins must be removed from the guide and cap. The cap then is removed. Afterward, the cap must be replaced and the stop pins replaced.
Other vent caps are known that are removable from the kelly valve in the event that tools must be passed downward through the kelly valve. A vent cap of this type is described in U.S. Pat. No. 4,364,407. Unfortunately, a wireline tool is required in order to remove the cap from the valve and then to replace it later.
A need exists for improved mud saver valves that can more effectively resist wear from abrasive drilling muds. A need also exists for an improved knockout cap that can be easily replaced and does not require stop pins or other connectors to hold it in place during operation.
The present invention provides a mud saver valve that features an outer housing or sub that retains upper and lower valve pistons. The pistons are reciprocably disposed within the housing and coordinate to provide a check valve though which fluid, such as drilling mud, is permitted to flow in one direction under pump pressure. Both the upper and lower valve pistons are provided with apertured plates that can be aligned in order to selectively open or close fluid passages defined by the apertures.
The valve configuration generates largely laminar flow through the valve. Turbulence is minimized because the direction of flow is not changed by the valve components.
In the preferred embodiment described here, the upper piston is disposed within the housing so that axial movement of the upper valve piston within the housing will also rotate the upper valve piston within the housing. In the described embodiment, a camming action is provided to rotate the upper piston within the housing and close the ports. The plates are secured within the piston sleeves using a keying arrangement. The plates are readily replaceable.
In operation, the spring causes axial movement of the piston sleeves within the housing and, thus, angular rotation of the plates with respect to one another, thereby opening a plurality of fluid flow ports to permit flow therethrough.
The invention also describes a frangible knockout vent cap that is readily replaceable and self-securing. The cap permits venting of excessive downhole pressure.